In the this application we propose to build on our experience with Pemetrexed (NCT01450384) and develop a novel drug therapy for non-small cell lung carcinoma (NSCLC) combining the standard of care drug Pemetrexed with approved phosphodiesterase-5 (PDE5) inhibitors. Pemetrexed synergizes with approved PDE5 inhibitors to kill NSCLC cells in vitro and in vivo. Sildenafil (Viagra), vardenafil (Levitra), and tadalafil (Cialis) are inhibitors of PDE5, which regulate cGMP levels and induce expression of nitric oxide synthase (NOS), increasing NO and ONOO- levels in tumor cells. Pemetrexed was developed as an inhibitor of thymidylate synthase but which was subsequently shown to inhibit the enzyme AICART. Aim 1. Determine in detail the molecular mechanisms by which PDE5 inhibitors and Pemetrexed interact to kill NSCLC cells. We hypothesize that PDE5 inhibitor and Pemetrexed treatment interact to abolish AKT, mTOR and p70 S6K activities leading to elevated levels of toxic autophagy and reduced expression of c-FLIP-s, BCL-XL and MCL-1. We hypothesize that PDE5 inhibitor and Pemetrexed treatment increases PERK/eIF2? phosphorylation also leading to suppression of c-FLIP-s, MCL-1 and BCL-XL expression. This results in higher free BAX and BAK (activity) and elevated levels of untethered Beclin1 which also facilitates the induction of toxic autophagy. We will determine the mechanisms by which sildenafil overcomes Pemetrexed resistance in NSCLC cells. We also hypothesize that PDE5 inhibitors: (a) increase iNOS levels which inactivates mTOR and p70 S6K and; (b) inhibit PTPase activity promoting death receptor CD95 tyrosine phosphorylation, ligand independent receptor activation and DISC formation. Aim 2. Determine using animal models whether PDE5 inhibitors enhance the therapeutic efficacy of Pemetrexed in NSCLC tumors. We will make use of transgenic and athymic animal models of NSCLC. Aim 2 will validate our in vitro data with NSCLCs using animal models with human lung cancer cell lines and mouse genetic models of lung cancer. We will determine whether sildenafil/Pemetrexed treatment chemo-sensitizes lung cancer tumors in vivo (to cisplatin).